1. Field of the Invention
The present invention relates to a light guide plate utilizing a subwavelength grating for controlling light transmission, and a backlight module for a liquid crystal display using such a light guide plate.
2. Description of Prior Art
Liquid crystal displays (LCDs) commonly use a backlight module to provide illumination. Referring to FIG. 6, a conventional backlight module includes a light source 101, a light guide plate 105, a reflective plate 103, a diffusion plate 107 and a prism plate 109. The light source 101 is typically a cold cathode fluorescent lamp (CCFL). The light guide plate 105 distributes light beams from the light source 101, to provide substantially planar illumination to the LCD.
Light beams typically enter the light guide plate 105 at a light incident surface. The light beams may propagate between a bottom surface and a light emitting surface toward an opposite end surface of the light guide plate 105 by total internal reflection, or may be output through the light emitting surface directly. Further, the bottom surface includes structures such as dots formed thereon or facets cut therein and arranged in a pattern. Light beams encountering any of these structures are diffusely or specularly reflected, so that they are emitted through the light emitting surface. However, when the light beams enter the light guide plate 105 through the light incident surface, part of the light beams are reflected by the light incident surface and lost. This reduces the light utilization efficiency of the light guide plate 105.
Referring to FIG. 7, Chinese Pat. No. 02249983 issued on Nov. 19, 2003 discloses a light guide plate (designated with the numeral 2). The light guide plate 2 comprises a substrate 20 having a light incident surface 21, a light emitting surface 22 adjacent to the light incident surface 21, a bottom surface 23 opposite to the light emitting surface 22, and side surfaces 24, 25 and 26. The light incident surface 21 and the light emitting surface 22 are provided with anti-reflection films (not labeled), and the bottom surface 23 and the side surfaces 24, 25 and 26 are provided with reflective films (not labeled). When light beams from a light source (not shown) are incident on the light incident surface 21 of the light guide plate 2, most of the light beams pass through the light incident surface 21, and relatively few light beams are reflected by the light incident surface 21. This reduces loss of light and enhances the light utilization efficiency of the light guide plate 2.
However, the light guide plate 2 has the following disadvantages and shortcomings. Firstly, the anti-reflection film is based on the principle of multi-light interference, and therefore can only provide anti-reflection in a narrow wavelength band. That is, light outside the narrow wavelength band is still reflected by the anti-reflection film. Thus the anti-reflection film has limited efficacy in reducing loss of light at the light incident surface 21. Secondly, the light incident surface 21 is generally closely adjacent to the light source, which may for example be a CCFL. When operating, the light source produces heat, which raises a temperature of the anti-reflection film. As a result, the reflection capability of the anti-reflection film is reduced. Thirdly, the anti-reflection film merely allows light beams to pass therethrough, but does not scatter the light beams. This can result in dark regions (not shown) being created on the light emitting surface 22 close to the light incident surface 21. That is, the illumination provided by the light emitting surface 22 is not uniform.
It is desired to provide a backlight module having a light guide plate which overcomes the above-described problems.